There is conventionally a photolithography technique in a technique for forming the three-dimensional face structural body such as a micro coil described in e.g., Japanese Patent No. 3132727. An exposure controlling photomask able to continuously control the light amount of transmitted light is used in this photolithography technique. There are the following photomasks in the exposure controlling photomask.
A first exposure controlling photomask is a mask for controlling the light amount of the transmitted light by laminating a light interrupting film described in the prior art of Japanese Patent Laid-Open Nos. 27636/1994 and 237625/1999 as shown in FIG. 13 in a stepwise shape. A second exposure controlling photomask is a mask for controlling the light amount of the transmitted light by adjusting the numerical aperture of a light interrupting area by a dot pattern (or hole) selectively drawn in a light interrupting film described in Japanese Patent Laid-Open Nos. 7115/1997 and 14725/1993 as shown in FIG. 14. In a third exposure controlling photomask, a transfer die having a die face having a convexo-concave portion of a curved surface shape is prepared as reported in Japanese Patent Laid-Open No. 237625/1999, and is pressed against a resin layer for adjusting the light transmitting amount on a mask substrate. The convexo-concave portion of the curved surface shape is transferred and formed on this resin layer, and this resin layer is next hardened by irradiating an ultraviolet ray, etc. The third exposure controlling photomask is a mask for controlling the light amount of the transmitted light by the film thickness of the convexo-concave portion of the curved surface shape in the resin layer formed in this way.
However, the first exposure controlling photomask has the stepwise shape with respect to the shape of the convexo-concave portion on the surface. Accordingly, when this exposure controlling photomask is manufactured, stepped portions of the convexo-concave portion are formed every one step. Therefore, it is necessary to repeatedly perform the photolithography for coating, exposing and developing photosensitive resin so that cost is raised. Further, since the convexo-concave portion on the surface has the stepwise shape, the photosensitive resin on the substrate intended to be transferred in this state as it is also becomes a convexo-concave portion of the stepwise shape. An overcoat layer must be formed by coating and burning an overcoat agent on the surface of the photosensitive resin to form the convexo-concave portion of the curved surface shape. However, it was difficult to control the photosensitive resin layer in the shape of the convexo-concave portion of the curved surface shape in the coating and burning processes.
In the second exposure controlling photomask, the light amount of the transmitted light depends on the size of the dot pattern (or hole). Therefore, it was difficult to control the size of the photosensitive resin in its transversal direction and the film thickness.
The third exposure controlling photomask has the convexo-concave portion of the curved surface shape smooth in comparison with the first and second exposure controlling photomasks so that the smooth convexo-concave portion of the curved surface shape can be also transferred to a photosensitive material.
In accordance with Japanese Patent Laid-Open No. 237625/1999, the pitch between top portions of a convex portion is about 1 to 100 times, preferably about 25 times the height of the convexo-concave portion of the curved surface shape. For example, when the height of the convexo-concave portion of the curved surface shape is 1.2 microns, the pitch between the top portions of the convex portion is written as about 30 microns. In the method for transferring the die by mechanical pressing, it is difficult to stably form the concavo-convex shape having a narrower pitch.
The conventional forming method of a micro lens will next be explained.
The micro lens is arranged as a solid state image pickup element every pixel, and is used to raise sensitivity by effectively converging light incident to each pixel to a light receiving element.
The conventional forming method of the micro lens will be explained in accordance with the processes of FIGS. 15(1) to 15(4).
A lens material film 13 is deposited on a flattened film 12 formed on a passivation film 11 of the solid state image pickup element. Thereafter, a resist film 14 is deposited on the entire surface (FIG. 15(1)). Subsequently, the resist film 14 is exposed and developed so that a separate independent pattern is formed every pixel. Concretely, patterning is performed such that each resist film 14 is located on a light receiving element 15 (FIG. 15(2)). Next, the resist film 14 is shaped in a convex curved surface shape (convex lens) by heating-melting processing (reflow) (FIG. 15(3)). Further, a micro lens 16 is formed by etching-back the resist film 14 and the lens material film 13 by the mixing gas of oxygen and a gas having a deposition property, and transferring the above convex curved surface shape to the lens material film 13 (FIG. 15(4)).
The other reference numerals and signs shown in FIG. 15 will be explained. Reference numerals 15, 17, 18 and 19 respectively designate a light receiving element, a semiconductor substrate, a vertical transfer electrode constructed by polycrystal silicon, and a light interrupting film constructed by aluminum.
In the above conventional forming method of the micro lens, the resist is shaped by heating and melting the resist to form the resist in the lens shape. However, the lens shape able to be formed is limited by only heating temperature and heating time, and it is difficult to control the fine convexo-concave portion of the curved surface shape.
In consideration of the above problems of the prior art, an object of the present invention is to provide an exposure controlling photomask and a production method thereof in which a light interrupting film on the exposure controlling photomask can be formed in the convexo-concave portion of a smoother curved surface shape in noncontact, and the pitch between top portions of a convex portion can be set to a narrower pitch.
Another object of the present invention is to provide a forming method for once transferring the lens shape to the resist by using the above exposure controlling photomask, and next etching-back a foundation material together with this resist, and transferring the lens shape to the foundation material.
Another object of the present invention is to provide a lens formed by irradiating light controlled in the exposure amount to photo-setting resin by using the above exposure controlling photomask, and its forming method.